A battery, which is mounted on a motor-operated vehicle such as an electric vehicle (EV) or a hybrid vehicle (HV) to supply electric power to a motor of the vehicle, needs a high voltage of, for example, about 300V. For this reason, the battery is configured as an assembled battery including multiple battery cells, each of which has a cell voltage of a few volts, connected in series. A lithium ion battery cell, which has been widely used in recent years, has a high cell voltage. Therefore, when the assembled battery is constructed with the lithium ion battery cells, the total number of battery cells in the assembled battery can be reduced, so that the size of the assembled battery can be reduced.
However, if each battery cell is not used within a predetermined cell voltage range between its minimum effective voltage and its maximum effective voltage, troubles such as a significant reduction in capacity of the battery cell and abnormal heat generation in the battery cell may occur. Further, if the battery cells have different cell voltages due to variations in their capacity, an error of a voltage of the assembled battery with respect to its target voltage may become large. For this reason, an equalization device for monitoring voltages of battery cells and equalizing the voltages has been demanded.
JP-A-2012-23848 corresponding to US 2013/0162213 discloses an equalization device having an equalization switch provided for each battery cell.
In addition to the equalization switches, the conventional equalization device has a level shift circuit which is provided for each equalization switch and operates on a power supply voltage produced by voltages of adjacent multiple battery cells. The level shift circuits are accumulated from a low potential side to a high potential side. The equalization device applies a drive voltage, which is level-shifted to the high potential side, between control voltages of the equalization switch in accordance with an equalization signal inputted with respect to a ground potential, for example.
In the conventional equalization device, if a connector connecting the equalization device and the assembled battery is disconnected, or if a power supply voltage or a ground potential of a control circuit, which receives the equalization signal and supplies the equalization signal to the level shift circuit, is lost, an operation of the level shift circuit becomes undefined, i.e., the drive voltage becomes undefined. As a result, an operation of the equalization becomes undefined.